Why can't I ping a network printer from a specific computer only on the same network?

After a lot of research on the web, I found a partial solution to the problem. It seems that this particular network printer (HP OfficeJet 8500 pro) does have a problem if there is also a 5 GHz network present. I turned it off on the router, and it suddenly works (I can ping and print from all computers).

However, this does not explain why previously it was only not possible to ping from computer A, but it worked from all other computers. Also, for future reference, I'd like to know how to further troubleshoot this kind of problems... any suggestions are welcome!